This invention relates generally to a method and apparatus for measuring position and orientation of features of a part or item. More specifically, the present invention relates to a method and apparatus for measuring position and orientation of features of a part or item, using a portable coordinate measuring machine (CMM).
The methodology used for measuring sheet metal features is complex due to the typically curved and complex nature of these features. The complexity of these features is compounded by the thinness of the metal sheets which makes it difficult to properly measure the position of edges using conventional measurement techniques. One of the two methods most commonly used today is check fixtures which are assemblages of posts, rests, clamps, etc. onto which the sheet metal part is placed and visual examination is made to seek variations between the hard fixture and the sheet metal part. This method is typically nonanalytical and is prone to significant setup error. It is also fundamentally a subjective process. The other method, which is the most commonly used for quality control of sheet metal parts, is the checking of a percentage of the parts on a computer controlled CMM. The use of a manual CMM is extremely infrequent. The reasons for this are explained as follows.
Since a CMM probe requires compensation for its probe thickness, it is necessary to establish vectors, normal to surfaces, and/or edges in order to define a direction of probe compensation. This process usually entails numerous hits. A hit is defined as the measurement of a location of the probe. Numerous hits are required in order to define surface vectors; for example, three hits on a surface to define a plane and, therefore, the normal to surface; two hits near an edge to define a normal to an edge. This multiplicity of hits is time consuming and makes the process prone to system crashes. A system crash on a computer controlled CMM is usually caused by variations in the part for which the programmed CMM motion is not adapted. This will result in unexpected probe contact with the surface (i.e., crashes) and, therefore, require reprogramming of the system. The multiple hits and their planning is also an extremely complex process, usually requiring off-line CMM simulation software and significant trial and error.
Further, it will be appreciated that everything in the physical world occupies volume or space. Position in a space may be defined by length, width and height which, in engineering terms, is often called an X, Y, Z coordinate. The X, Y, Z numbers represent the dimensions of length, width and height or three dimensions. Three-dimensional objects are described in terms of position and orientation; that is, not just where an object is but in what direction it points. The orientation of an object in space can be defined by the position of three points on the object. Orientation can also be described by the angles of alignment of the object in space. The X, Y, and Z coordinates can be most simply measured by three linear scales. In other words, if you lay a scale along the length, width and height of a space, you can measure the position of a point in the space.